1. IntroductionFire has always been an important ecological role in shaping the components of landscapes. In addition, it also plays a key role in the conservation and management of biodiversity in regions which have coevolved with this disturbance (e.g. the Mediterranean and the Chaco Region). In semiarid landscapes, seasonality and vegetation characteristics promote the fire occurrence (Bravo et al. 2001; Bravo 2010). However, within the global context of climatic and land use changes, fire regimes have been altered, becoming in increasing severity and extension events (Bravo 2010).Plants have been biochemically prepared to respond to environmental changes through the biosynthesis of secondary metabolites. Indeed, physical stress produced by fire promotes the biosynthesis of these compounds which provide resistance to vegetation and predisposes it to new fire events which could be even more severe. Recent studies have highlighted the effect of foliar organic chemistry on flammability, focusing on volatile organic compounds (VOCs) such as terpenoids, which are characterized by reducing ignition temperatures in both foliage and litter (Ormeño et al. 2009; Page et al. 2012; Bowmanet al. 2014; Della Rocca et al. 2017). Furthermore, these compounds have an important role on plant defense to herbivores and pathogens (Page et al. 2012; Della Rocca et al. 2017; Diaz-Guerra et al. 2018).The role of chlorophylls, carotenoids, phenolic compounds and tannins in response to fire have still been poorly investigated. Chlorophyll is one of the main indicators of photosynthetic capacity and physiological status of plants (Cambrón-Sandoval et al. 2011; Callejas et al. 2013; Jaramillo-Salazar et al. 2018). Indeed, quantification of photosynthetic pigments contributes to determine the plant behavior during its development cycle under stress conditions. (Goel et al. 2004; Huot et al. 2007; Cruz et al. 2009). The function of carotenoids beyond being an accessory pigment in photosynthesis is the protection against photosensitization due to chlorophylls (Nisar et al. 2015). In addition, these compounds are considered important elements of information storage in response to environmental changes (Allred & Snyder 2008; Esteban et al.2015). Furthermore, foliar persistence is an important trait on plant flammability (Santacruz-García et al. 2019) which influences total chlorophyll and carotenoids concentrations due to the foliar absence of deciduous species during the dry and winter season (Takashimaet al. 2004).Phenolic compounds have an important role in plant defense against herbivore (Ganthaler et al. 2017; Diaz-Guerra et al.2018).These compounds are considered as stress bioindicators due to its high sensitivity to changes in environmental conditions (Lagadicet al. 1997). Indeed, Cannac et al. (2009) reported an increase in the synthesis of phenolic compounds in response to prescribed burnings in Pinus laricio. In addition to the above mentioned functions, tannins reduce the digestibility of plant tissues through the formation of indigestible complexes with dietary protein in herbivores (García 2015). Phenolic metabolites are recognized as strong antioxidant agents which protect cells against free radical damage (Linet al. 2018). Hence, the renewed interest in native plants as a source of many biologically active compounds and natural antioxidants in phytomedicine (Asif 2015).Vegetation strategies for tolerance to environmental disturbances should be studied as a challenge within the current scenario of climate and land use change. The traditional ecological approach considers that fire tolerance is exclusively related to post-disturbance vegetation regeneration strategies, which include the resprouting ability from bud banks and/ or germination through seed banks (Clarke et al.2013). However, this work proposed the evaluation of plant tolerance to fire from a biochemical approach which had not been previously explored by any study in the fire ecology area. The link between the burnt biomass during a fire event and the biochemical response of plant could be considered as a plant tolerance to fire indicator. Considering that fire tolerance is directly related to the fire severity (Bran et al. 2007), and fire severity is related to the intensity and duration of the event and it indicates the degree to which vegetation has been affected by this disturbance; the burnt biomass is a measure of fire severity (Montorio et al. 2007).The Chaco Region forests have been strongly affected by anthropic disturbances as wildfires, livestock, logging and mechanical treatments to silvopastoral systems (Bravo et al. 2014; Ledesma et al. 2018). Forest management strategies include tools such as mechanical treatments and prescribed burning to shrub clearing and to improve pastures for livestock (Talamo & Caziani 2003; Kunst et al. 2012; Grau et al. 2015; Santacruz-Garcia et al. 2019). However, the response of native vegetation to this disturbance possibly reveals an adaptation and tolerance to fire (Bowman et al. 2014; Bravo et al. 2014). The knowledge of the effect of foliar chemical composition on vegetation response to fire contributes to understand the patterns of postfire recovery of native vegetation in burnt forests. In addition, it could provide tools to wildland fire management strategies (Cannac et al. 2009; Ormeño et al.2009; Jaureguiberry 2012; Della Rocca et al. 2015, 2017).The objectives of this work were a) to evaluate the effect of experimental burns in the concentrations of photosynthetic pigments as chlorophylls and carotenoids, phenolic compounds and tannins in leaves of six woody species, b) to determine the temporal dynamics of the biosynthesis of chlorophylls and secondary metabolites in response to fire and c) to establish the relationship between the plant tolerance to fire (according to the burnt biomass during experimental burns) with the biochemical response of plants. We evaluated the following hypothesis: a) Fire promotes a variation in the concentration of plant metabolites (Cannac et al. 2007, 2009; Ormeño et al. 2009; Della Roccaet al. 2017). The total chlorophyll and carotenoid contents decrease (Carter & Knapp 2001; Jaramillo-Salazar et al. 2018), whereas, phenolic compounds and tannins increase after fire events (Cannac et al. 2007), b) The stability and post fire recovery is dependent on biochemical responses as antioxidants biosynthesis that promote the plant tolerance and its resilience to fire regime (Bowmanet al. 2014; Enright et al. 2014), c) Vegetation affected by fire increases the biosynthesis of natural bioactive compounds with potential applications in health and agroforestry sectors (Karjalainenet al. 2009; Edwin-Wosu et al. 2017) and d) The biochemical response of plants to fire is related to the amount of burnt biomass (Hoffmann & Moreira 2002; Keeley et al. 2011).This study represents the first effort to evaluate a set of metabolites involved in plant responses to fire since a strict biochemical approach. The most studies evaluate response to fires without considering ecophysiological behavior. The knowledge of plant biochemical adaptation strategies to fire could contribute to understand the mechanism of tolerance and resilience of plants in natural environments. A temporary variation on the concentration of chlorophylls and secondary metabolites in response to fire could reveal the role of these compounds as bioindicators of plant tolerance to fire.